This program project grant in venereal diseases has included four major areas of research in the past year: (1) The specific serum requirements for phagocytosis of type 4 gonococci have been delineated. The principal serum opsonin is immunoglobulin G. IgM does not promote phagocytosis. IgG mediated phagocytosis is amplified by complement via the alternate pathway. (2) The effect of benzyl penicillin on the synthesis and cell morphology of the cell envelope of Neisseria gonorrhoeae was examined. Penicillin immediately halted murein synthesis; lipids and lipopolysaccharide were shed in the extracellular medium. Subsequent lysis could be prevented by growing cells at low pH and high concentrations of magnesium. (3) The initial interaction between virulent Treponema pallidum and human mononuclear phagocytes is being studied in vitro. Treponemes attach to the leukocyte surface membranes with a polar orientation and are not ingested. Attached spirochetes retain motility and virulence but active treponemal metabolism is not needed for attachment to occur. Antitreponemal IgG antibody is not opsonic. (4) The detailed behavior of the growth characteristics of a nonpathogenic Trichomonas vaginalis under different oxygen concentrations has been studied. The inhibition of growth by oxygen is concentration dependent although there is a slow continuous growth even in the presence of oxygen. The mechanisms of oxygen tolerance (presence of enzymes detoxifying the reduction products) are being explored.